removed google plus link seeing as google plus has been shut down for a little bit now (#3935)

README.md

@@ -20,7 +20,6 @@ Please read [CONTRIBUTING](/CONTRIBUTING.md). If you're new to GitHub, [welcome]
 
 + [Share on Twitter](http://twitter.com/intent/tweet?text=https://github.com/EbookFoundation/free-programming-books%0AFree%20Programming%20Books)
 + [Share on Facebook](https://www.facebook.com/share.php?u=https%3A%2F%2Fgithub.com%2FEbookFoundation%2Ffree-programming-books&p[images][0]=&p[title]=Free%20Programming%20Books&p[summary]=)
-+ [Share on Google Plus](https://plus.google.com/share?url=https://github.com/EbookFoundation/free-programming-books)
 + [Share on LinkedIn](http://www.linkedin.com/shareArticle?mini=true&url=https://github.com/EbookFoundation/free-programming-books&title=Free%20Programming%20Books&summary=&source=)
 + [Share on Telegram](https://t.me/share/url?url=https://github.com/EbookFoundation/free-programming-books)